HUNTER: BEHAVIOR OF LARVAL ANCHOVY 



Master, 1971); dry weight 1 rotifer = 0.16 fig 

 of which 92.2% is organic material (Theilacker 

 and McMaster, 1971); and the assumption of 

 100% digestive efficiency. The estimate was 

 converted from unit weight to length by the 

 length-dry weight relationship of log W = 3.3237 

 log L — 3.8205 given by Lasker et al (1970). 



The metabolic requirement given above was 

 adjusted for feeding success by increasing it in 

 proportion to the number of prey missed during 

 feeding as predicted by the feeding success equa- 

 tion presented previously. The density of food 

 required for survival was estimated by dividing 

 the food requirement adjusted for feeding suc- 

 cess by the volume of water searched in 10 hr. 

 The average of the photographic and visual esti- 

 mates of volume searched was used to calculate 

 the volume searched by larvae. A 10-hr feeding 

 period was chosen purely as a convenience be- 

 cause the actual duration of daily feeding periods 

 is unknown. 



The density of rotifers required to meet met- 

 abolic requirements decreased exponentially 

 with size (Figure 12). First feeding larvae, 3 

 days old (3.5 mm) require 105 rotifers or the 

 caloric equivalents/liter during a 10-hr feeding 

 period whereas 10-day-old larvae (5.9 mm) re- 

 quire only 34 rotifers/liter. Older larvae require 

 much lower food densities primarily because 

 feeding success increases exponentially with age. 



Density estimates based on rotifer equivalents 

 probably underestimate the prey density re- 

 quired during the first few days of feeding be- 

 cause a smaller prey is needed by most anchovy 

 larvae during this time. For example, Theilack- 

 er and McMaster (1971) found that only 12% 

 of anchovy larvae survive to 19 days old when 

 Brachionus was the only food, whereas 40 to 

 50% survived if in addition to Brachionus the 

 dinoflagellate, Gymnodinium, was present dur- 

 ing the first few days of feeding. To obtain a 

 better estimate of food density required by lar- 

 vae during the first days of feeding I estimated 

 the caloric value of a single Gymnodinium cell 

 and recalculated the density required in terms 

 of dinoflagellate equivalents. The caloric value 

 of a single Gymnodinium cell was estimated 

 from the carbon content (Mullin and Brooks, 

 1970) and by assuming that the carbon content 



200r 



AGE (days) 



15 20 



30 



n 2.000 



q: 



Ul 



^ 100- 



> 



3 



o 



o 



-I 

 < 



o 



06 



08 10 



LENGTH (cm) 



12 



14 



Figure 12. — Estimate of the density of prey in rotifer 

 caloric equivalents/liter (left ordinate, solid line) and 

 in dinoflagellate equivalents (right ordinate, dashed line) 

 required to meet metabolic requirements of larval an- 

 chovy of 0.3-1.5 cm. Lines are based on average esti- 

 mates. The shapes of the two curves are the same, 

 but they differ in elevation by a constant factor of 17. 



represented 40% of the organic material and 

 that the caloric value of Gymnodinium was 5,000 

 cal/g of organic material. According to this cal- 

 culation the caloric value of a single Brachionus 

 is about 17 times that of a single Gymnodinium 

 cell. Thus larvae feeding on dinoflagellates re- 

 quire about 17 times the prey density (dashed 

 line Figure 12) than do those feeding on rotifers 

 (solid line). Nearly all larvae are able to feed 

 on Brachionus by age 5 days ; thus, the food den- 

 sity requirement for the majority of the larvae 

 shifts from 1,790 dinoflagellates/liter at age 3 

 days (the first day of feeding) to 48 rotifer 

 equivalents/liter at age 5 days. 



835 



